BOM item is not valid

What causes this issue?

You want to:
Assign an alternative or a variant to, or
Copy
a BOM item on a key date.
However, the item is invalid on this key date.

System Response

The system is unable to carry out the operation.

How to fix this error?

Select a key date on which the item is valid.

Error message extract from SAP system. Copyright SAP SE.